In a TOF-MS, accelerated ions are injected into a flight space where no electric field or no magnetic field is present, and the ions are separated by their mass to charge ratios with the flight time of the ions in the flight space. For the ion source of a TOF-MS, an ion trap device is used in many cases.
As shown in FIG. 4, a typical ion trap device 1 is composed of a ring electrode 11, and a pair of end cap electrodes 12, 13 placed opposing each other with the ring electrode 11 between them. Usually, an RF (radio frequency) voltage is applied to the ring electrode 11 to form a quadrupole electric field in the ion trapping space 14 surrounded by the electrodes 11, 12, 13, whereby ions are trapped within the ion trapping space 14. In one case, ions are generated outside of the ion trap device 1 and introduced into it, and in another case they are generated within the ion trap device 1. The theory of such an ion trapping method is described in detail in, for example, “Quadrupole Storage Mass Spectrometry” by R. E. March and R. J. Hughes, John Wiley & Sons, 1989, pp. 31–110.
A wide variety of samples are analyzed by such mass analyzers, and the range of mass to charge ratio to be analyzed depends on the sample. In an ion trap device, ions are not only trapped and stored in the ion trapping space, but also manipulated in various processes such as cooling their vibrational motion, selection of ions with specific mass to charge ratio and excited for collisional dissociation to perform structural analysis of the sample. The amplitude of the RF voltage is controlled so that the trapping potential appropriate for each process is established.
When ions are to be analyzed in the TOF-MS, the RF voltage applied to the ring electrode 11 is stopped after various processings as mentioned above are done and object ions are prepared in the ion trapping space 14. Then an ejecting voltage is applied to the end cap electrodes 12, 13 to form an ion ejection electric field in the ion trap device. Owing to the ion ejection electric field, ions are accelerated and ejected through a hole 13a in an end cap electrode to the TOF-MS, where a mass analysis of the ions are achieved.
The RF voltage applied to the ring electrode 11 just before ions are ejected from the ion trapping space 14 differs depending on the mass to charge ratio of the ejected ions and the processings that the ions have undergone in the ion trap device 1. For example, as shown in FIG. 5, when the RF voltage is stopped using a high-speed switch at tc, the actual voltage of the ring electrode 11 (which will be referred to as “the ring voltage”) does not instantaneously become that of the end cap electrodes 12, 13 (which will be referred to as “the end cap voltage”, and is zero in the case of FIG. 5), but gradually approaches it with an oscillation (which is called a “ringing”), because an RF resonance coil and an RF resonance capacitor are connected to the ring electrode 11. That is, a certain period of time is necessary until the ring voltage subsides to the end cap voltage.
If, before the ring voltage subsides to the end cap voltage, an ion ejecting voltage is applied to the end cap electrodes 12, 13 to eject ions from the ion trap device 1 to the TOF-MS 3, the ion ejection electric field in the ion trap device 1 has a variation from the calculated target field, and there arises an error in the initial kinetic energy of the ejected ions. Since the amplitude of the ringing depends on the amplitude of the RF voltage before the stop, variation in the ejection electric field when ions are ejected, a certain period after the stop time tc, also changes with it.
If the error in the initial kinetic energy is small, the width of the peak changes little in the mass spectrum, and it does not affect the resolution in the mass to charge ratio. But the error in the kinetic energy affects the flight time of the ions, which results in a shift in the peak in the mass spectrum and makes it difficult to accurately determine the mass to charge ratio of the ions.
On the other hand, if enough time is allotted from the stop time tc to the ion ejecting time (i.e., enough time is taken until the ringing subsides), the ring voltage stabilizes and the above problem does not arise. In this case, however, the state where no quadrupole electric field exists in the ion trapping space lasts longer, so that ions may disperse before the ion ejection electric field is formed. This decreases the number of ions to be used in the analysis, and deteriorates the sensitivity of the analysis.
An object of the present invention is therefore to provide a mass analyzer and a mass analyzing method in which the shift of a peak or peaks in a mass spectrum is minimized while maintaining a high analyzing sensitivity, and the mass to charge ratio can be determined at high accuracy.
In the first aspect of the present invention, a mass analyzer comprises:
an ion trap device including an ion trapping space surrounded by a plurality of electrodes;
a time-of-flight mass analyzer for determining a mass to charge ratio of ions ejected from the ion trapping space;
a trapping voltage generator for generating an ion trapping RF voltage to at least one of the plurality of electrodes;
an ejecting voltage generator for generating an ejecting voltage to at least one of the plurality of electrodes to form an ion ejection electric field for ejecting ions trapped in the ion trapping space; and
a controller for stopping the ion trapping RF voltage at a timing when ions are trapped in the ion trapping space and the ion trapping RF voltage is at a predetermined phase, and for applying the ion ejecting voltage a predetermined period after the ion trapping RF voltage is stopped.
In the second aspect of the present invention, a mass analyzing method comprises the steps of:
trapping ions in an ion trapping space surrounded by a plurality of electrodes by applying an ion trapping RF voltage to at least one of the plurality of electrodes;
stopping the ion trapping RF voltage at a timing when ions are trapped in the ion trapping space and the ion trapping RF voltage is at a predetermined phase; and
applying an ion ejecting voltage to at least one of the plurality of electrodes for forming an ion ejection electric field to eject ions trapped in the ion trapping space to a time-of-flight mass analyzer a predetermined period after the ion trapping RF voltage is stopped.
In the present invention, in both aspects, the phase and the timing are predetermined under the condition that the voltage of the electrode or electrodes to which the ion trapping RF voltage was applied becomes almost the same the predetermined period after the ion trapping RF voltage is stopped at the predetermined phase, irrespective of the amplitude of the ion trapping RF voltage when it is stopped.
In the present invention, in both aspects, the electrode to which the ion trapping RF voltage is applied is normally the ring electrode, and the electrode to which the ion ejecting voltage is applied is normally the end cap electrodes. Other voltage configuration is of course possible in the present invention.
In the present invention, the ion ejection electric field is formed at the timing when the voltage of the ring electrode is the same as that of the end cap electrodes while the voltage of the ring electrode is still ringing after the ion trapping RF voltage is stopped. Since the frequency of the ringing is low, the voltage of the end cap electrodes can be regarded as constant while the ions are being ejected. Thus the kinetic energy of the ions ejected from the ion trapping space to the TOF-MS does not vary, and the flight time of the ions in the TOF-MS does not vary, either. This brings the peak of the ions to the same place in the mass spectrum, and makes it possible to determine the mass to charge ratio of the ions precisely.
If the amplitude of the ion trapping RF voltage before it is stopped is changed according to the mass to charge ratio of the ions to be analyzed, the amplitude of the ringing after the stop of the RF voltage also changes. The inventor of the present invention has found out that, if the ion trapping RF voltage is stopped at a certain phase, the voltage of the ring electrode becomes the same as the voltage of the end cap electrodes or, at least, becomes a certain fixed voltage after a certain time period irrespective of the amplitude of the ringing. The phase and the time period depend on the electric parameters of the electric circuit around the ion trap including the ion trap itself and its power source, but they are determined if the constitution of the device is fixed. Thus the phase and the time period can be experimentally determined beforehand, and the controller can use the values to stop the ion trapping RF voltage and to start applying the ion ejecting voltage.
In the present invention, by precisely controlling the stopping time of the the ion trapping RF voltage to come to a predetermined phase of the RF voltage, the voltage of the ring electrode can be adjusted to be the same as that of the end cap electrodes a certain time period after the ion trapping RF voltage is stopped, irrespective of the amplitude of the ion trapping RF voltage when it is stopped. Thus, by applying the ion ejecting voltage at such a timing, the initial kinetic energy of the ejected ions does not vary, and a precise determination of the mass to charge ratio of the ions becomes possible.
Even if the voltage of the ring electrode cannot be brought to be the same as that of the end cap electrodes, it suffices if the voltage of the ring electrode can be brought to a certain predetermined value, because the same ion ejection electric field can be formed by adjusting the ejecting voltage of the end cap electrodes by the difference between the predetermined value and the end cap voltage. In this case also a precise determination of the mass to charge ratio of the ions becomes possible.